Rejected online payment

I have a wifi contract with telkom since January 2025, I changed my banking details on the 26th of June and the person who was assisting me told me that I must make an online payment as they are going to deduct end of July. I made an online payment on the 30th of June, yesterday when I checked my total due for end July it showed I am in arrears. I requested a statement today only to find out they rejected my payment on the 3rd of July, how do I terminate my contract with this company as I am fed up with their *****ed ways